Regulation and function of the mammalian central nervous system is governed by a series of interdependent receptors, neurons, neurotransmitters, and proteins. The neurons play a vital role in this system, for when externally or internally stimulated, they react by releasing neurotransmitters that bind to specific proteins. Common examples of endogenous small molecule neurotransmitters such as acetylcholine, adrenaline, norepinephrine, dopamine, serotonin, glutamate, and gamma-aminobutyric acid are well known, as are the specific receptors that recognize these compounds as ligands (“The Biochemical Basis of Neuropharmacology”, Sixth Edition, Cooper, J. R.; Bloom, F. E.; Roth, R. H. Eds., Oxford University Press, New York, N.Y. 1991).
In addition to the endogenous small molecule neurotransmitters, there is increasing evidence that neuropeptides play an integral role in neuronal operations. Neuropeptides are now believed to be co-localized with perhaps more than one-half of the 100 billion neurons of the human central nervous system. In addition to being found in humans, neuropeptides have been discovered in a number of animal species. In some instances, the composition of these peptides is remarkably homogenous among species. This finding suggests that the function of neuropeptides is vital and has been impervious to evolutionary changes. Furthermore, neuropeptides, unlike small molecule neurotransmitters, are typically synthesized by the neuronal ribosome. In some cases, the active neuropeptides are produced as part of a larger protein that is enzymatically processed to yield the active substance. Based upon these differences, compared to small molecule neurotransmitters, neuropeptide-based strategies may offer novel therapies for the treatment of CNS diseases and disorders. Specifically, agents that affect the binding of neuropeptides to their respective receptors or that ameliorate responses that are mediated by neuropeptides are potential therapies for diseases associated with neuropeptides.
There are a number of afflictions that are associated with the complex interdependent system of receptors and ligands within the central nervous system; these include neurodegenerative diseases, affective disorders such as anxiety, depression, pain and schizophrenia, and affective conditions that include a metabolic component, namely obesity. Such conditions, disorders, and diseases have been treated with small molecules and peptides that modulate neuronal responses to endogenous neurotransmitters.
One example of this class of neuropeptides is neuropeptide Y (NPY). NPY was first isolated from porcine brain (Tatemoto, K. et al. Nature 1982, 296, 659) and was shown to be structurally similar to other members of the pancreatic polypeptide (PP) family such as peptide YY (PYY), which is primarily synthesized by endocrine cells in the gut, and pancreatic polypeptide, which is synthesized by the pancreas. NPY is a single peptide protein that consists of thirty-six amino acids containing an amidated C-terminus. Like other members of the pancreatic polypeptide family, NPY has a distinctive conformation that consists of an N-terminal polyproline helical region and an amphiphilic alpha-helix joined by a characteristic PP-fold (Vladimir, S. et al. Biochemistry 1990, 20, 4509). Furthermore, NPY sequences from a number of animal species have been elucidated and all show a high degree of amino acid homology to the human protein (more than 94% in rat, dog, rabbit, pig, cow, sheep) (see Larhammar, D. in “The Biology of Neuropeptide Y and Related Peptides”, Colmers, W. F. and Wahlestedt, C. Eds., Humana Press, Totowa, N.J. 1993).
Endogenous receptor proteins that bind NPY and related peptides as ligands have been identified and distinguished, and several such proteins have been cloned and expressed. Five different receptor subtypes [Y1, Y2, Y4 (PP), Y5, Y6 (a pseudogene in human)] are recognized based upon binding profile, pharmacology, and cDNA sequence (Kaga, T. et al. Peptides 2001, 22, 501-506; Wahlestedt, C. et al. Ann. N.Y. Acad. Sci. 1990, 611, 7; Larhammar, D. et al. J. Biol. Chem. 1992, 267, 10935; Wahlestedt, C. et al. Regul. Pept. 1986, 13, 307; Fuhlendorff, J. U. et al. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 182; Grundemar, L. et al. J. Pharmacol. Exp. Ther. 1991, 258, 633; Laburthe, M. et al. Endocrinology 1986, 118, 1910; Castan, I. et al. Endocrinology 1992, 131, 1970; Gerald, C. et al. Nature 1996, 382, 168; Weinberg, D. H. et al. J. Biol. Chem. 1996, 271, 16435; Gehlert, D. et al. Curr. Pharm. Des. 1995, 1, 295; Lundberg, J. M. et al. Trends Pharmacol. Sci. 1996, 17, 301). All NPY receptor proteins belong to the family of so-called G-protein coupled receptors (GPCRs).
NPY itself is the archetypal substrate for the NPY receptors and its binding can elicit a variety of pharmacological and biological effects in vitro and in vivo. When administered to the brain of live animals (intracerebroventricularly (icv) or into the amygdala), NPY produced anxiolytic effects in established animal models of anxiety such as the elevated plus-maze, Vogel punished drinking, and Geller-Seifter's bar-pressing conflict paradigms (Heilig, M. et al. Psychopharmacology 1989, 98, 524; Heilig, M. et al. Regul. Pept. 1992, 41, 61; Heilig, M. et al. Neuropsychopharmacology 1993, 8, 357). Thus, compounds that mimic NPY are postulated to be useful for the treatment of anxiolytic disorders.
The immunoreactivity of NPY is notably decreased in the cerebrospinal fluid of patients with major depression and those of suicide victims (Widdowson, P. S. et al. J. Neurochem. 1992, 59, 73), and rats treated with tricyclic antidepressants displayed significant increases of NPY relative to a control group (Heilig, M. et al. Eur. J. Pharmacol. 1988, 147, 465). These findings suggest that an inadequate NPY response may play a role in some depressive illnesses, and that compounds that regulate the NPY-ergic system may be useful for the treatment of depression.
It is known that the anxiolytic properties of NPY are mediated through postsynaptic Y1 receptors, whereas presynaptic Y2 receptors negatively control the release of NPY and other cotransmitters (e.g. GABA). Consequently, antagonism of the Y2 receptor may lead to enhanced GABAergic and NPY-ergic effects and Y2 receptor antagonists should prove useful in the treatment of depression and anxiety.
NPY improved memory and performance scores in animal models of learning (Flood, J. F. et al. Brain Res. 1987, 421, 280) and therefore may serve as a cognition enhancer for the treatment of neurodegenerative diseases such as Alzheimer's Disease (AD) as well as AIDS-related and senile dementia.
Elevated plasma levels of NPY were present in animals and humans experiencing episodes of high sympathetic nerve activity such as surgery, newborn delivery, and hemorrhage (Morris, M. J. et. al. J. Auton. Nerv. Syst. 1986, 17, 143). Thus, chemical substances that alter the NPY-ergic system may be useful for alleviating migraine, pain, and the condition of stress.
NPY also mediates endocrine functions such as the release of luteinizing hormone (LH) in rodents (Kalra, S. P. et. al. Front. Neuroendrocrinol. 1992, 13, 1). Since LH is vital for mammalian ovulation, a compound that mimics the action of NPY could be useful for the treatment of infertility, particularly in women with so-called luteal phase defects.
NPY is a powerful stimulant of food intake; as little as one-billionth of a gram, when injected directly into the CNS, caused satiated rats to overeat (Clark, J. T. et al. Endocrinology 1984, 115, 427; Levine, A. S. et al. Peptides 1984, 5, 1025; Stanley, B. G. et al. Life Sci. 1984, 35, 2635; Stanley, B. G. et al. Proc. Nat. Acad. Sci. U.S.A. 1985, 82, 3940). Thus NPY is orexigenic in rodents but not anxiogenic when given intracerebroventricularly and so antagonism of neuropeptide receptors may be useful for the treatment of diabetes and eating disorders such as obesity, anorexia nervosa, and bulimia nervosa.
Recently, a key role of presynaptic hypothalamic Y2 receptor was suggested in central coordination of energy homeostasis and bone mass regulation (Herzog, H. et al. Drug News & Perspectives 2002, 15, 506-510). Studies analyzing Y2 receptor knockout mice have started to unravel some of the individual functions of this receptor subtype. Y2 receptor knockout mice showed a reduced body weight despite an increase in food intake, possibly due to the lack of the feedback inhibition of the postprandially released PYY3-36 (Batterham, R. L. et al. Nature 2002, 418, 650-654). The Y2 receptor knockout mice also showed a significant increase in bone formation (Baldock, P. A. J. Clin. Invest. 2002, 109, 915-921). Specific deletion of the Y2 receptor in the hypothalamus in adult conditional Y2 receptor knockout mice also caused an increase in bone formation.
A direct link between NPY signaling and regulation of ethanol consumption was suggested by the demonstration that NPY over-expression in mice reduced ethanol self-administration, whereas the knockout of NPY expression increased ethanol self-administration (Thiele et al. Nature 1998, 396, 366-369). Studies have also indicates that NPY Y2 is involved in the neurobiological responses to ethanol and other drugs of abuse. Thiele and coworkers (Neuropeptides, 2004, 38(4), 235-243; Peptides 2004, 25(6), 975-983) described the low ethanol consumption of Y2 receptor knockout mice, as well as their increased voluntary water consumption. Recently, it has been demonstrated that icv administration of BIIE0246, a selective NPY Y2 antagonist, dose dependently reduced ethanol self-administration in rats (Thorsell et al. Neurosci. Lett. 2002, 332, 1-4). Therefore, modulators of NPY Y2 may allow for the treatment of alcohol and drug abuse.
Additionally, NPY Y2 antagonists have been suggested for the prevention of cardiovascular disease, for example, sudden death due to cardiac arrhythmias, post-myocardial infarction, or heart failure (See: Intl. Pat. Appl. Publ. WO 02/083137, Oct. 24, 2002).
Grouzmann and coworkers described a peptide-based ligand, T4-[NPY 33-36], which showed considerable affinity (IC50=67 nM) for the NPY Y2 receptor (Grouzmann, E., et al. J. Biol. Chem. 1997, 272, 7699-7706). BIIE0246 also bound to the NPY Y2 receptor with significant affinity (IC50=3.3 nM) (Doods, H., et al. Eur. J. Pharmacol. 1999, 384, R3-R5). However, the therapeutic potential for these compounds is limited due to their peptide-like composition and elevated molecular weight.
Thus, there remains a need for potent NPY Y2 modulators with desirable pharmaceutical properties.